In a wireless communication system, a mobile station is enabled to communicate with an access station of a wireless communication network by means of a connection via a radio interface.
The radio resources, which are available for a particular wireless communication system, can be used in different simultaneous connections without interference by splitting the radio resources up into different channels. For example, in Frequency Division Multiple Access (FDMA), different frequencies are employed for different connections. In Time Division Multiple Access (TDMA), available radio resources are divided into frames, each frame comprising a predetermined number of time-slots. To each connection, a different time-slot may then be assigned in each frame. In Code Division Multiple Access (CDMA), different codes are used in different connections for spreading the data over the bandwidth.
A wireless communication system typically comprises a plurality of fixed stations as access stations, each enabling a communication with mobile stations located in one or more sub-areas served by the fixed station. A sub-area can be for instance a cell of a cellular communication system or a sector of a sectorized wireless communication system. It is to be understood that in case reference is made to a cell in the following, the same applies to a sector.
Using a plurality of cells allows reusing the same channels in various cells. In this case, however, it has to be ensured that interference is kept sufficiently low not only within a respective cell, but also between different cells of the system.
In cellular FDMA/TDMA systems, intra-cell interference is minimized by transmitting signals at different time-slots and/or at different frequency channels in the same cells.
Inter-cell interference can be managed by defining a co-channel reuse distance. That is, the same time-slots/frequencies are only used by cells having a certain reuse distance to each other, the reuse distance being selected such that the co-channel interference between these cells is reduced sufficiently by the path loss of transmitted signals.
However, in order to exploit the available radio resources optimally or to avoid excessive usage of bandwidth, a low frequency-reuse, that is, a very small reuse distance, may be preferred in a FDMA/TDMA system. A small reuse distance implies that the same frequency is reused in cells close to each other. The current working assumptions of 1 Gbps maximum data rate for a 4 G cellular system and 200 Mbps maximum data rate for a 3.9 G cellular system, for instance, result in a high frequency bandwidth requirement. Only a low frequency re-use, optimally a frequency re-use of ‘one’, allows achieving reasonable multi-operator scenarios with limited total bandwidth availability. A frequency re-use of ‘one’ implies that all cells may use all available frequency channels.
A small reuse distance may lead to severe inter-cell interference, in particular at the cell edges. For cellular systems having low frequency reuse, inter-cell interference, or co-channel interference if the same frequency channel is used, is thus a critical issue. In this case, a smart Radio Resource Management (RRM) is essential for keeping inter-cell interference at an acceptable level by controlling the transmission power levels employed in each cell.
The impact of the employed transmission power levels on inter-cell interference is illustrated in FIGS. 1a and 1b. 
FIG. 1a presents a part of a cellular communication system, in which three base stations BS0 110, BS1 120 and BS2 130 serve a respective cell, cell 0 111, cell 1 121 and cell 2 131. The base stations 110, 120 130 are located by way of example at a corner of the associated cell 111, 121, 131. The cells 111, 121 and 131 are represented according to the hexagonal model.
FIG. 1b presents an exemplary power sequence employed by one of the base stations, indicating that the transmission power employed by a base station is not constant but may be varied for instance between a high power level p1, a middle power level p2 and a low power level p3. The transmission power may be varied either as a function of time within one time period T, like a radio frame, or as a function of frequency within the employed frequency band F.
FIG. 1a indicates in each cell 111, 121, 131 the respective coverage 112, 122, 132 that is achieved with the lowest transmission power p3 of the power sequence of FIG. 1b. It can be seen as long as all base stations 110, 120 130 use this lowest transmission power p3, there is no interference between the cells. Transmission power p3, however, does not enable a full coverage of the cells, a full coverage requiring the highest transmission power p1 in each cell 111, 121, 131. A transmission power p1, however, does not only guarantee a full coverage of a cell, but will also cause interference in the neighboring cells.
It has been proposed for non-CDMA type systems that transmissions at high power levels in different cells are shifted to different timings. Due to such a time-shift in a low frequency-reuse environment, inter-cell interference can be managed in a way that worst interference situations, resulting from simultaneous transmissions at peak power in different cells, can be avoided.
It also has to be considered that for a continuous coverage, pilot and broadcast channels must be receivable over the whole area of a cell. For an environment in which time-based frequency re-use is generated by specifying timeslot-specific power restrictions for each base station, it has been proposed that the pilot channel, training sequence and system information transmissions take place at fixed positions in the high-power timeslots of a radio frame. This principle is illustrated in FIG. 2.
FIG. 2 presents three diagrams. The first diagram indicates the transmission power P over time t for cell 0. The second diagram indicates the transmission power P over time t for cell 1. The third diagram indicates the transmission power P over time t for cell 2. It can be seen that the transmission power varies stepwise from a highest value to a lowest value and back to a highest value again. Further, the respective highest values in the three power sequences are shifted against each other in time. In a synchronized system, it can be guaranteed that this shift is maintained. The time slots to which the highest transmission power is assigned are used in each of the cells for transmitting pilot channel, training sequence and system information. Thus, this information is made available at different times for neighboring base stations. After initial power-on, a mobile station may seeks the preferred cell by looking for the known pilot transmissions. With the defined distribution to the highest transmission power time-slots, it is ensured that a mobile station will be able to receive the system information regardless of its location in a cell.
It has not been proposed so far, however, how to design such power sequences.